Here's my backward compatible Unity 4.3-only "undo class" for you to use.

Unity Engine
1

Hi, I made this for our plugins Master Audio and Killer Waves, to add full Undo support - the new kind that came with Unity 4.3. I figured I’d make it easy for whoever wants to do the same thing.

#if UNITY_3_0 || UNITY_3_1 || UNITY_3_2 || UNITY_3_3 || UNITY_3_4 || UNITY_3_5
   // Undo API 
#else
    using UnityEditor;
#endif
using UnityEngine;
using System.Collections;

public static class UndoHelper {
    public static void CreateObjectForUndo(GameObject go, string actionName) {
        #if UNITY_3_0 || UNITY_3_1 || UNITY_3_2 || UNITY_3_3 || UNITY_3_4 || UNITY_3_5 || UNITY_4_0 || UNITY_4_1 || UNITY_4_2
            // No Undo API 
        #else 
            // New Undo API
            Undo.RegisterCreatedObjectUndo(go, actionName);
        #endif
    }

	public static void SetTransformParentForUndo(Transform child, Transform newParent, string name) {
		#if UNITY_3_0 || UNITY_3_1 || UNITY_3_2 || UNITY_3_3 || UNITY_3_4 || UNITY_3_5 || UNITY_4_0 || UNITY_4_1 || UNITY_4_2
			// No Undo API 
		#else 
			// New Undo API
			Undo.SetTransformParent(child, newParent, name);
		#endif
	}

	public static void RecordObjectPropertyForUndo(Object objectProperty, string actionName) {
		#if UNITY_3_0 || UNITY_3_1 || UNITY_3_2 || UNITY_3_3 || UNITY_3_4 || UNITY_3_5 || UNITY_4_0 || UNITY_4_1 || UNITY_4_2
			// No Undo API 
		#else 
			// New Undo API
			Undo.RecordObject(objectProperty, actionName);
		#endif
	}

	public static void RecordObjectsForUndo(Object[] objects, string actionName) {
		#if UNITY_3_0 || UNITY_3_1 || UNITY_3_2 || UNITY_3_3 || UNITY_3_4 || UNITY_3_5 || UNITY_4_0 || UNITY_4_1 || UNITY_4_2
		// No Undo API 
		#else 
			// New Undo API
			Undo.RecordObjects(objects, actionName);
		#endif
	}

	public static void DestroyForUndo(GameObject go) {
		#if UNITY_3_0 || UNITY_3_1 || UNITY_3_2 || UNITY_3_3 || UNITY_3_4 || UNITY_3_5 || UNITY_4_0 || UNITY_4_1 || UNITY_4_2
			GameObject.DestroyImmediate(go);
		#else 
			// New Undo API
			Undo.DestroyObjectImmediate(go);
		#endif
	}
}
2

To use it to provide undo for a field change in a custom Inspector class, replace things like this:

kill.HitPoints = EditorGUILayout.IntSlider("Hit Points", kill.HitPoints, 1, Killable.MAX_HIT_POINTS);

with this:

var newHP = EditorGUILayout.IntSlider("Hit Points", kill.HitPoints, 1, Killable.MAX_HIT_POINTS);
f (newHP != kill.HitPoints) {
     UndoHelper.RecordObjectPropertyForUndo(kill, "change Hit Points");
     kill.HitPoints = newHP;
}

and voila, you have undo. But only in Unity 4.3. Other versions are ignored. If you wanted to provide undo for those other versions, you can modify the UndoHelper class accordingly.

I hope this helps someone!